1. Field of the Invention
The present invention relates generally to a pressure control system for an automatic power transmission for an automotive vehicle. More specifically, the invention relates to a pressure control system for an electronically or electrically controlled automatic power transmission which can enhance durability of an electrically operated actuator.
2. Description of the Background Art
U.S. Pat. No. 3,621,735 discloses a pressure control system for an automatic power transmission, which employs a duty controlled solenoid. The duty controlled solenoid receives a periodic signal having a given duty cycle and controls a ratio of an opening period and a closing period of a fluid flow restricting orifice. By periodically opening and closing the flow restriction orifice, a magnitude of signaling pressure for a pressure regulator valve is adjusted. The pressure regulator valve is responsive to the signaling pressure to vary line pressure in a hydraulic circuit of the automatic power transmission.
In such conventional construction, the solenoid is responsive to the periodic signal to perform a signaling pressure adjusting operation irrespective of the status of the hydraulic circuit. For instance, even when the line pressure is not supplied, the solenoid becomes active in response to the periodic signal to perform adjustment of the signaling pressure. In such case, the plunger of the solenoid contacts against the orifice in absence of the working fluid to cause a shortening of the effective life of the solenoid.